The process by which a photoresist mask is formed on a semiconductor wafer involves first coating the wafer with a thin layer of photoresist and then exposing the photoresist with the desired pattern. The photoresist layer is then developed. This involves a number of heating and cooling steps. In a typical process, the clean wafer is subjected to a dehydration bake at 100.degree.-150.degree. C. to remove any moisture that has accumulated and thereby promote the adhesion of photoresist to the wafer. In addition, a chemical such as hexamethyldisilazane (HMDS) may be used at this point to improve adhesion between the photoresist and the wafer. A thin layer of photoresist is then applied to the wafer by spinning. The wafer is subjected to a "soft" bake at 90.degree.-120.degree. C. to create a firm bond with the photoresist and dry the photoresist by driving off any photoresist solvents. The wafer can then be transferred to the exposure system.
After the photoresist has been exposed, the wafer is subjected to a post-exposure bake at 60.degree.-120.degree. C. to solidify the pattern, and the photoresist is then developed, forming a pattern on the surface of the wafer. Following the developing step, the wafer is subjected to a "hard" bake at 130.degree.-160.degree. C. to dry the wafer and increase the adhesion of the photoresist to the silicon surface. After each of the foregoing baking steps, the wafer is cooled to 18.degree.-25.degree. C. in order to assure a uniform process.
All of these steps must be performed in a clean room which is temperature and humidity controlled and substantially free of dust and other particulate matter. At present, the most common system for performing this process uses a track to transport the wafer to successive stages of the process. Track systems have limited flexibility, since the wafers are locked in a fixed order. In a less common type of arrangement, various processing modules are positioned on either side of a central track, and the wafers are transported from module to module by a robot which moves back and forth along the central track. Such an arrangement is extremely wasteful of floor area since no processing occurs in the central track area. In addition, the throughput of this arrangement is limited by the speed with which the robot can move from one end of the system to the other.
In addition, in these prior art systems, if a single robot arm handles all the substrates, only one substrate may be serviced at any one time. Thus if two substrates are finished baking at the same time, only one of the substrates can be removed from the hot plate. The other substrate may be overbaked.
Also, in prior art systems, since a robot arm's end effector removes substrates from a hot plate, the end effector usually heats up over a period of time. When the hot end effector picks up a room temperature substrate having a recently coated photoresist layer, the photoresist layer may be heated non-uniformly in areas touched by the end effector. This may result in the photoresist coating being thicker in areas where the substrate was heated by the end effector. A 1.degree. C. variation in temperature can result in 20 .ANG. variation in thickness. Such variations are unacceptable since modern substrate processing must yield coatings of uniform thickness of 0.5 micron with variations no more than approximately 10 .ANG..
Furthermore, in the prior art, coiled vacuum lines are often used to supply vacuum for holding substrates in position. These vacuum lines, which extend through the equipment, may get in the way of robots, belts and other moving equipment and get chewed up and broken.
Silicon wafers which are being processed in a semiconductor fabrication facility are typically held in cassettes when they are not undergoing processing. A standard cassette has been adopted according to specifications issued by the Semiconductor Equipment Manufacturers Institute (SEMI), and the standard cassette is used almost universally in the semiconductor industry. The cassettes containing silicon wafers must be transported from one production step to the next throughout a fabrication facility. Typically, these cassettes are carried inside plastic boxes. Once a box with a cassette arrives at a particular piece of process equipment, an operator opens the box, removes the cassette and places the cassette into the equipment.
The cassettes are positioned inside the plastic transport boxes with the "H" bar forward and the flat surfaces of the wafers oriented vertically. The force of gravity keeps the wafers seated in the cassette.
Most processing equipment requires that the cassettes be introduced into the equipment with the "H" bar down and the wafers oriented horizontally. This configuration allows the equipment to access the individual wafers for processing. Thus, when the cassette is placed into the equipment, it must be rotated after it has been removed from the transport box. This rotation is typically done by an operator grasping the cassette from the side with both hands and rotating his or her wrists 90.degree. while placing the cassette into the equipment. The repeated rotation of the filled cassettes, which typically weigh over seven pounds, has resulted in numerous repetitive use injuries, such as carpal tunnel syndrome, among equipment operators.
The processing equipment is increasingly being automated with robots. To keep the equipment safe for operators, it must be enclosed at all times. This requirement adds to the difficulty of cassette input and output. When an operator is inserting a cassette into or removing a cassette from the equipment, for example, a barrier must separate the operator and any potentially hazardous mechanisms in the equipment. The only alternative is to shut the equipment down at these times, but doing this detracts from the production rate of the equipment. To maximize the output of processed wafers, it is important that the equipment operate continuously. This means that the equipment should not be stopped in order to change cassettes.
Thus, there is a need for a cassette input/output unit which overcomes all of these problems, i.e., avoids the need for the operator to rotate the cassette when it is inserted into the equipment, provides a safety barrier at all times which prevents the operator from inadvertently making contact with robots or other hazardous mechanisms inside the equipment, and allows the equipment to be operated without interruption while cassettes are being exchanged.
These problems are overcome in a clustered photolithography system in accordance with this invention.